In a multilayer printed wiring board formed by laminating interlaminar insulating layers and conductor layers, the conductor layers are divided into a power layer, a signal layer and a shield layer in accordance with use purpose and functioned individually. Among them, the power layer is usual to be a flat conductor pattern having a large surface area.
On the other hand, the multilayer printed wiring board is produced through an additive process by alternately building up a resin insulating layer and a conductor layer on a substrate and then electrically connecting the conductor layers to each other through a viahole formed in the resin insulating layer. In the production process of such a multilayer printed wiring board, therefore, there is a tendency that a solvent is liable to be retained in the interlaminar insulating layer disposed on a lower side of the conductor layer.
For example, when an air permeable resin layer such as a resist is formed on the interlaminar insulating layer, such a residual solvent is evaporated and removed by a heat treatment such as a drying treatment conducted at a plating step or the like, so that such a solvent does not particularly cause a problem.
However, when an impermeable metal layer such as a conductor layer is formed on the interlaminar insulating layer, it is very difficult to remove the above residual solvent by evaporation. Particularly, when the power layer having a flat pattern as a conductor pattern among the conductor layers is formed on the resin insulating layer through additive process, the residual solvent is retained between the power layer and the resin insulating layer as a vapor and there is a problem easily causing so-called "swelling of conductor layer".
Further, such a "swelling of conductor layer" degrades the adhesion property between the resin insulating layer and the conductor and hence causes the lowering of the interlaminar insulating property.
It is, therefore, an object of the invention to provide a multilayer printed wiring board capable of solving the above problems resulting from the residual solvent, and to particularly propose a conductor pattern of a power layer capable of preventing the swelling of the conductor layer and effectively acting to improve the adhesion property between the resin insulating layer and the conductor.